Regression

Regression is a defense mechanism, where there is a reversion to an earlier stage of development in the face of unacceptable thoughts or impulses. For an example an adolescent who is overwhelmed with fear, anger and growing sexual impulses might become clingy and start exhibiting earlier childhood behaviors he has long since overcome, such as bed-wetting. An adult may regress when under a great deal of stress, refusing to leave their bed and engage in normal, everyday activities.

When confronted by stressful events, people sometimes abandon coping strategies and revert to patterns of behavior used earlier in development. Anna Freud called this defense mechanism regression, suggesting that people act out behaviors from the stage of psychosexual development in which they are fixated. For example, an individual fixated at an earlier developmental stage might cry or sulk upon hearing unpleasant news.

Behaviors associated with regression can vary greatly depending upon which stage the person is fixated at:

• An individual fixated at the oral stage might begin eating or smoking excessively, or might become very verbally aggressive.
• A fixation at the anal stage might result in excessive tidiness or messiness.

sources: http://psychcentral.com/lib/15-common-defense-mechanisms/0001251

            http://psychology.about.com/od/theoriesofpersonality/ss/defensemech_10.htm